Volatile abundances and oxygen isotopes in basaltic to dacitic lavas on mid-ocean ridges: The role of assimilation at spreading centers

Abstract

a b s t r a c t a r t i c l e i n f oMost geochemical variability in MOR basalts is consistent with low-to moderate-pressure fractional crystallization of various mantle-derived parental melts. However, our geochemical data from MOR highsilica glasses, including new volatile and oxygen isotope data, suggest that assimilation of altered crustal material plays a significant role in the petrogenesis of dacites and may be important in the formation of basaltic lavas at MOR in general. MOR high-silica andesites and dacites from diverse areas show remarkably similar major element trends, incompatible trace element enrichments, and isotopic signatures suggesting similar processes control their chemistry. In particular, very high Cl and elevated H 2 O concentrations and relatively light oxygen isotope ratios (~5.8‰ vs. expected values of~6.8‰) in fresh dacite glasses can be explained by contamination of magmas from a component of ocean crust altered by hydrothermal fluids. Crystallization of silicate phases and Fe-oxides causes an increase in δ 18 O in residual magma, but assimilation of material initially altered at high temperatures results in lower δ 18 O values. The observed geochemical signatures can be explained by extreme fractional crystallization of a MOR basalt parent combined with partial melting and assimilation (AFC) of amphibole-bearing altered oceanic crust. The MOR dacitic lavas do not appear to be simply the extrusive equivalent of oceanic plagiogranites. The combination of partial melting and assimilation produces a distinct geochemical signature that includes higher incompatible trace element abundances and distinct trace element ratios relative to those observed in plagiogranites.

Composition of the Oceanic Crust

“…O with decreasing Mg number is modest, less than 1% across the entire spectrum of compositions, consistent with earlier studies (e.g., Muehlenbachs and Byerly, 1982). Wanless et al (2011) argued that the weaker than expected correlation of d 18 O with MgO, particularly among the rare andesitic and dacitic lavas found along mid-ocean ridges, was due to assimilation of hydrothermally altered oceanic crust, which has low d 18 O. There are no statistically significant correlations between stable and radiogenic isotope ratios in this dataset, although this may, in some cases, reflect the paucity of data.…”

Rheology and the Fe3+–chlorine reaction in basaltic melts

“…Both chlorine and fluorine are found in MORB melts, with up to 0.07 wt% Cl in erupted basaltic glass and up to 0.31 wt% (~0.25 mol% Cl 2 O -1 ) in basaltic andesite glasses erupted at mid-oceanic ridges (Wanless et al, 2011). Similar amounts of F (0.02 wt %, ~0.03 mol% F 2 O -1 ) are found in mid-Atlanitic ridge glasses (Byers et al, 1986) and (0.06 wt% F) in the "MORB-like" basalts from the Galapagos Archipelago plume (Koleszar et al, 2009 (Black et al, 2012).…”

Sill to surface: Linking young off-axis volcanism with subsurface melt at the overlapping spreading center at 9°03′N East Pacific Rise

“…2, and sample descriptions and compositional variations are given in Supplemental Table 1. Sample major and trace element compositions have been published elsewhere (Wanless et al, 2010(Wanless et al, , 2011.…”

“…We selected 22 of these samples for 238 U-230 Pb isotopic age dating. These samples span a range of tholeiitic volcanic rock compositions, including ferrobasalt, basaltic andesite (or basaltic icelandite), andesite (or icelandite; cf., Carmichael, 1964), and dacite (Wanless et al, 2010(Wanless et al, , 2011; and, tectonic settings including the axial graben, the ridge flank, and the large, axis-parallel pillow ridge and flat-topped seamount that overlie the westernmost extent of the wide melt lens (Fig. 1).…”